Treatment of oily wastewater generated by cruise ships has been regarded as one of worldwide challenges for eco-environmental protection for centuries. In general, oil spill pollution can be divided into three types: that caused by maritime transportation, that caused by coastal industrial activity and that occurs on inland water areas. Oil spill pollution occurring on inland water areas (including rivers and lakes) refers to the pollution of the inland water areas resulting from direct dumping of the untreated bilge water generated in the engine room of the cruise ships. This kind of pollution has engendered severe negative impact upon the fragile eco-system of inland waters in China. The negative effect has emerged in Dianchi Lake, Yunnan Province, China, and currently all diesel-engined ships are banned to enter the lake by local authority. The pollution caused by the same reason also occurred in the Three Gorges area of the Yangtze River. Besides, in July of 2007, Nanjing Maritime Agency (of Jiangsu Province, China) stipulated that drain outlets of all ships cruising in Nanjing reach of the Yangtze River should be closed. However, this method cannot solve the oil spill pollution once and for all. In China, the huge number of diesel-engined ships will definitely produce much bilge water. If effective methods are not adopted in treating the bilge water, it would pollute inland waters in one way or another even though the direct dumping is prohibited. Therefore, the crucial strategy of dealing with the wastewater pollution is to find an effective way for treating the bilge water. The characteristics of the bilge water requires special treatment: first, the oil content at different levels of the bilge water changes so greatly that it varies from several mg/L at the lower level to several hundred mg/L at the upper level. This consequently increases the difficulty of treatment. Second, engine rooms of most ships are spatially compact. The oil separation apparatus should be small and highly effective. Third, all ships work in the state of movement. Therefore, the oil separation apparatus should be designed with high and stable working performance in the state of tossing and pitching. The apparatus in this invention is the very device designed for treatment of bilge water generated by cruise ships (for both maritime and inland water transportation). Fourth, according to Resolution MEPC.107(49) stipulated by International Maritime Organization—“Revised guidelines and specifications for pollution prevention equipment for machinery space bilges of ships”—the anti-pollution apparatus should be capable to tolerate the passage of 100% concentration of specific oil medium for five minutes while no harm occurs to the apparatus itself. This special requirement poses a demanding challenge for the design of the anti-pollution apparatus of this kind.